The principal objective of the research being supported by Grant HL 6950 is to extend our knowledge about those metabolic properties of red cells of man and of other vertebrates which are responsible for their viability and their function in the transport of oxygen. We are particularly interested in finding out what adaptive and phylogenetic variations of red cell metabolism may have evolved in different species. Additional major aims are to describe the nature of the very extensive changes which take place in the biochemistry of the red cell: a) during embryonic and early post-natal development and b) during early differentiation and maturation in the particular hematopoietic organ or tissue being used.